Pile underpinning is a specialized and robust method used to strengthen or stabilize the foundations of an existing building, particularly when the original foundation is no longer adequate or the underlying soil conditions are poor. This technique involves extending the foundation's depth by installing piles to transfer the building's weight to deeper, more stable ground layers.
Understanding Pile Underpinning
At its core, pile underpinning works by bypassing weak, unstable, or compressible soil near the surface and transferring the structural load to stronger, deeper bearing strata. This is achieved by installing individual piles (long, slender columns, typically made of concrete or steel) beneath the existing foundation. These piles are either driven into the ground or bored and filled with concrete, effectively creating new, deep foundation elements.
Piled Raft Underpinning: A Comprehensive Solution
One of the most extensive and effective forms of pile underpinning is the piled raft underpinning system. This method is a preferred choice for scenarios requiring significant foundational reinforcement.
Key Characteristics & Application:
Piled raft underpinning systems are commonly used when an entire building needs to be underpinned. This makes them ideal for structures experiencing widespread settlement, significant ground movement, or when there's a need to considerably increase the building's load-bearing capacity, such as during extensions or changes in use.
The Underpinning Process
The implementation of a piled raft underpinning system involves a multi-step, systematic approach to ensure the comprehensive support of the entire structure:
- Removal of Internal Floors: To begin, the internal floors are completely removed. This crucial step provides essential access to the existing ground level beneath the building, allowing for the subsequent foundation work to proceed without obstruction.
- Pile Installation: Following floor removal, a grid of piles is installed. These piles are strategically placed across the building's footprint, driven or bored deep into the ground until they reach a suitable, stable load-bearing stratum. The "grid" ensures even distribution of future loads.
- Reinforced Concrete Raft Construction: Once the piles are in place, a reinforced concrete raft is then constructed over the complete floor level. This raft acts as a new, robust ground slab, effectively tying all the new piles together and creating a unified base for the entire building. Reinforcement bars within the concrete provide tensile strength and prevent cracking.
- Load Transfer and Support: The final and critical function of this system is that the newly constructed raft effectively begins picking up and fully supporting all external and internal walls. The building's weight is uniformly transferred through the raft to the grid of piles, which then safely transmit the loads to the deep, stable ground.
Benefits of Piled Raft Underpinning
The adoption of piled raft underpinning offers several significant advantages, especially for large-scale or severely affected structures:
- Enhanced Stability: Provides a robust and long-term solution against severe subsidence, preventing future foundation issues.
- Comprehensive Support: Unlike localized underpinning, it supports both internal and external walls, making it ideal for whole-building stabilization.
- Load Distribution: The concrete raft evenly distributes the building's weight across the grid of piles, minimizing stress concentrations.
- Versatility: Suitable for a wide range of challenging ground conditions, including expansive clays, soft soils, or areas with variable water tables.
- Increased Structural Integrity: Contributes significantly to the overall structural integrity and longevity of the building.
When is Underpinning Necessary?
Building owners often consider underpinning when they observe signs of structural distress, which can indicate foundation problems. Common indicators include:
- Cracks: Visible cracks in walls (especially diagonal ones), floors, or ceilings.
- Unevenness: Floors becoming noticeably uneven or sloping.
- Sticking Doors and Windows: Doors and windows that jam or no longer close properly due to frame distortion.
- Subsidence: A clear sinking or settlement of one part of the building.
- Ground Conditions: Changes in ground conditions (e.g., prolonged drought causing clay shrinkage, burst pipes, nearby excavation).
Pile underpinning, particularly the comprehensive piled raft system, provides a durable and effective solution to these complex foundation challenges, ensuring the long-term stability and safety of the structure.
